Postfix behind NAT / Real IP

i have a problem with my Postfix setup. The Postfix Server is connected to my local network, with a private address 10.0.0.10. On my gateway Server i use a iptables DNAT rule for Port 25. Everything works fine, but i can not see the Source IPs in the Postfix Logfile. So it looks like all emails come from the internal gateway server with the address 10.0.0.1.

What is the best Setup for an internal Postfix Server behind a Firewall wich have no public address? I will see the real client addresses in my Postfix log. (and for fight spam)